I agree they do have differences, but maybe the relative benefit/£ ratio is not the same as an amp or speakers. The jump from a £300 Dac to a £900 one is not as big sonically as making the same change to your donwstream kit. Having said that, the Chord QDB76 is a pretty special thing, but at £3k it should be; however, for me I am sure I will have got a bigger change from spending £1500 on a speaker change than spending the same on a DAC upgrade
Agree that different speakers make large differences to the sound, but sometimes it's just different rather than a real upgrade.
Also, speakers that are too good for the electronics reveal problems in the system.
I think subtle changes in the source can also make a big difference to how enjoyable/musical the result is in the long term. You can't really appreciate this by just flicking switches between DACs while listening to a track.
Totally agree about the amp though, if in doubt upgrade amp first!
Tube DACs sound wonderful if correctly designed and have a good quality valves used. I have a first hand experience with Tube DACs made on various chips such as Wolfson WM8741, Brown PCM1794, TDA1541 and ALL of them sounding very engaging that make you just seat and enjoy your music. Of course your other components need to help your DAC signal to be as least effected as possible to bring that dynamic and natural sound to your ears. When it comes to 16 Bit CD format to me TDA1541 is a King and all other Chips in Tube DAC design comes very close with a different flavour to every one of them, but the point that in the right Tube DAC design all of the major chips are capable to deliver a very musical sound.